The pictures shows how those who were considered to be heretics were literally regarded as demons who brought temptations and tried to make good Christians deviate from the path taught by God and by the Holly Scriptures.
This argument is quite clear in the image were there is a priest, or maybe abishop, who is making a big effort to stand still in between all those flying demons. He counts with the aid of God, who is represented with the angel, but still the task is very complicated and requires determination and strenght.
